Displacement detecting device by light retroreflectance having first and second retroreflecting units with a common light receiving unit

Abstract

A displacement detecting device includes a scale diffraction grating and a detecting head unit. The detecting head unit includes a light source, a first retroreflecting unit that retroreflectes positive first-order diffracted light of light diffracted by the scale diffraction grating, such that the retroreflected light enters the scale diffraction grating again, a second retroreflecting unit that retroreflectes negative first-order diffracted light of the light diffracted by the scale diffraction grating, such that the retroreflected light enters the scale diffraction grating again, and a light receiving unit that receives an interference signal. Each of the first retroreflecting unit and the second retroreflecting unit has a deflecting function of deflecting light incident on the corresponding retroreflecting unit by a predetermined angle and then emitting the light.

What is claimed is: 
     
       1. A displacement detecting device comprising:
 a scale diffraction grating; and 
 a detecting head unit that is disposed so as to be relatively movable with respect to the scale diffraction grating, and detects the amount of relative displacement to the scale diffraction grating, 
 wherein the detecting head unit includes
 a light source that emits light toward the scale diffraction grating, 
 a first retroreflecting unit that retroreflects positive s-order diffracted light of light diffracted by the scale diffraction grating, such that the retroreflected light enters the scale diffraction grating again, 
 a second retroreflecting unit that retroreflects negative s-order diffracted light of the light diffracted by the scale diffraction grating, such that the retroreflected light enters the scale diffraction grating again, and 
 a common light receiving unit that receives an interference fringe formed on the common light receiving unit by the light retroreflected by the first retroreflecting unit and diffracted by the scale diffraction grating and the light retroreflected by the second retroreflecting unit and diffracted by the scale diffraction grating, 
 
 each of the first retroreflecting unit and the second retroreflecting unit has a deflecting function of deflecting light incident on the corresponding retroreflecting unit by a predetermined angle and then emitting the light (wherein “s” is an integer of 1 or greater), and 
 a position of a point on the scale diffraction grating where the light retroreflected from the first retroreflecting unit enters is different from a position of a point on the scale diffraction grating where the light retroreflected from the second retroreflecting unit enters. 
 
     
     
       2. The displacement detecting device according to  claim 1 , wherein:
 each of the first retroreflecting unit and the second retroreflecting unit includes a corner cube and a wedge prism. 
 
     
     
       3. The displacement detecting device according to  claim 2 , wherein:
 the corner cube and the wedge prism are integrated. 
 
     
     
       4. The displacement detecting device according to  claim 1 , wherein:
 each of the first retroreflecting unit and the second retroreflecting unit includes a reflective mirror and two lenses having different focal lengths.